1. Field of the Invention
The invention is in the field of thin-film devices for conversion of light to electrical energy, such as in the fabrication of solar conversion modules.
2. Brief Description of the Background Art
Chalcopyrite semiconductors, such as thin films of copper-indium-diselenide (CuInSe2), copper-gallium-diselenide (CuGaSe2), and Cu(Inx,Ga1-x)Se2, all of which are sometimes generically referred to as CIGS, have become the subject of considerable interest and study for semiconductor devices in recent years. They are of particular interest for photovoltaic device or solar cell absorber applications because solar energy to electrical energy conversion efficiencies (on a total area basis) of 18.8% have been achieved in devices employing Cu(Inx,Ga1-x)Se2 with x approximately equal to 0.3 (see M. A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, and R. Noufi, Prog. Photovolt: Res. Appl. 7, 311-316 (1999)). This is quite high for current state-of-the-art solar cell technologies. It is generally believed by persons skilled in this art that the best electronic device properties, thus the best conversion efficiencies, are obtained when the mole percent of copper is about equal to the mole percent of the indium, the gallium, or the combination of the indium and gallium in the Cu(In,Ga)Se2 compound or alloy. The electrical band gap of CIGS can be varied between about 1.0 eV to 1.68 eV through variation of the Ga content. The selenium content will not generally be important to the electronic properties of the semiconductor if the growth conditions supply sufficient or excess selenium so that it comprises about 50 at. % of the Cu(In,Ga)Se2 compound to form the desired crystal lattice structures. Sulfur can also be, and sometimes is, substituted for the selenium, so the compound is sometimes referred to even more generically as Cu(In,Ga)(S,Se)2 to comprise all of those possible combinations. A preferred method for the production of large panels of these materials is physical vapor deposition (see, for example, Alan Delahoy, Juergen Bruns, Liangfan Chen, Masud Akhtar, Zoltan Kiss and Miguel Contreras, xe2x80x9cAdvances in Large Area CIGS Technologyxe2x80x9d 28th IEEE Photovoltaic Specialists Conference, Anchorage, Ak., Sep. 15-22, 2000). An extensive review of work in this area can be found in U.S. Pat. No. #Re.31,968 issued in 1985 to R. Mickelsen, et al.
The energy conversion efficiency of these devices depends critically on their composition. For example, for Cu(In,Ga)Se2 devices, it has been observed that the highest energy conversion efficiencies occur when the ratio of copper to indium plus gallium is from approximately 0.8 to approximately 0.95. At lower ratios the film resistivity is too high and for higher ratios, the films become too metallic in nature.
In physical vapor deposition processes, it is common to control the deposition rate of each constituent by controlling the temperature of that source. While it is possible to set up a production system to approximately reproduce the desired deposition rates based on individual measurements made before a deposition run, in order to achieve acceptable quality control, it is necessary to control these rates during deposition. This has been done on a laboratory scale, for example by optical absorption measurements in the vapor phase in order to infer flux rate (M. Powalla, et al., Proceedings of the 14th European Photovoltaic Solar Energy Conference, Barcelona, Spain, Jun. 30-Jul. 4, 1997, pp.1270-1273) and by mass spectrometer measurements of the depositing vapor (L. Stolt, et al., Proceedings of the 13th European Photovoltaic Solar Energy Conference, Nice, France, Oct. 23-27, 1995).
For commercial production, it would be desirable to be able to perform a non-contacting measurement of a deposited film and to be able to adjust the deposition condition so as to maintain optimum film properties.
It has been found to be possible to monitor the quality of Cu(In,Ga)Se2 thin films, as an example of chalcopyrite films, by making spectrophotometric measurements of light reflected from the film surface. This permits the result of non-contacting measurements of films in a continuous production environment to be fed back to adjust the production conditions in order to improve or maintain the quality of subsequently produced film. An advantage of this method is that it obtains information about the film directly, and does not require inference of film properties by sampling of the vapor streams. The latter is subject to calibration errors and inaccuracies due to variable loss of In from the growing film via a volatile selenide.
There are many ways of characterizing the color of light reflected from a surface. The tristimulus method measures the light reflected from the object using three sensors filtered to have the same sensitivity as the human eye. Another spectrophotometric method measures the intensity of the reflected light at many wavelengths and determines the position of the color in a three dimensional space (Lab space) specified by a red-green axis, a yellow-blue axis, and a black-white brightness axis.
While the above methods are useful in examining light reflected from a surface, it has been found that another useful quality function for CIGS films can be derived by measuring the surface reflectance at two discrete wavelengths and calculating the ratio between the two reflectances. In a physical vapor deposition process, this quality function is determined for each film as it emerges from the deposition apparatus. This quality function is compared to experimentally predetermined quality measures in a system processor and a control signal is derived and fed back to adjust the deposition conditions so as to drive the system toward improving subsequent product. This spectrophotometric determination is a single, rapid process that is easily adapted to the production environment.